Proponents of dry collection systems contend that the use of wet scrubbers for conditioning and cleaning pulverized coal fired steam generator flue gases merely converts an air pollution problem into a potential water pollution problem. As a result, the disposal of the fly ash fraction collected in the scrubber along with the treatment of the scrubbing liquor inventory are important considerations when designing a wet collection system for a given application.
Since environmental regulations prohibit indiscriminate contamination of wall water courses with suspended or dissolved solids contained in wet scrubber effluents, it is imperative that all scrubbers are designed to operate as near as possible within the parameters of a totally enclosed or "closed-loop" system. Enforcement of these regulations will contribute to an eventual abandonment of "once-through" scrubbing water circuits in favor of a "closed-loop" mode of operation. Before this can be accomplished, however, a more effective method of controlling the quantity and quality of the system blowdown liquors must be perfected.
In order to achieve a "closed-loop" mode of operation, the scrubbing liquor bleed-off or blowdown rate, apart from the dissolved solids removed from the system in the evaporator circuit, is limited essentially by the quantity of dissolved solids rejected from the system as mechanically entrained liquor in the ash pond or filter cake residue. Discounting the dissolved solids rejected from the system in the form of a chemical precipitate, the resulting blowdown rate is a function of the dissolved solids carrying capacity of the recycled scrubbing liquor. This, in turn, is influenced by the chemical content of the ash collected in the scrubber and by the solubility rates of the alkaline earth, silica and clay minerals present when the ash is intermixed and suspended in weak acid solutions formed in the scrubbing process. Experience has shown that approximately 20% of the sulfuric acid mist and SO.sub.2 contaminants in the flue gases will be absorbed in aqueous scrubbing solutions without additions of any chemical absorbents.
The findings of a recent water balance survey performed at an existing power plant scrubber installation indicated that a liquor blowdown rate equivalent to 12.57% of the total scrubbing liquor inventory was required in order to insure prolonged trouble-free scrubber operations. By controlling the pH of the scrubber effluent between 4.3 and 4.5, the total dissolved solids in the scrubbing liquor at steady state conditions exceeded concentrations of 4000 ppm with corresponding calcium hardness levels in the region of 2000 ppm as CaCO.sub.3. Any attempt to operate below the blowdown rate indicated above or at pH or dissolved solids levels in excess of these values, produced instantaneous scaling throughout the scrubbing liquor circuit with the most prevalent scaling occuring in the wet-dry zone area of the scrubbers. Of more significance, the 12.57% liquor blowdown rate reported is excessive by most standards and confirmed the need for tightening the scrubbing liquor circuit. In order to accomplish this and still function within the realm of existing governmental codes, the above stated dissolved solids and calcium hardness levels would have to be increased nearly twenty-fold. It is apparent that a feasible post chemical treatment method would be required for maintaining solids concentrations in the scrubbing liquor well below the levels required in a "closed-loop" mode of operation. Otherwise, severe operating problems would result from solids crystallizing out of suspension to form extremely hard-to-remove scale deposits in the scrubber proper.